1. Technical Field of the Invention
The present invention relates to a method for manufacturing an active matrix type liquid crystal display device that can suitably be employed in a portable telephone, a portable information terminal and the like. More particularly, it relates to a method for manufacturing an active matrix type liquid crystal display device that is able to prevent deterioration of images displayed thereon.
2. Description of the Related Art
An active matrix type liquid crystal display device comprises, for example, a TFT substrate using a thin film transistor (TFT) (formed in each pixel), as a switching element an opposing substrate adhered to the TFT substrate and a liquid crystal layer interposed between the TFT substrate and the opposing substrate.
FIG. 1 is a layout diagram illustrating a construction of a TFT substrate employed in a conventional liquid crystal display device and FIG. 2 is a cross sectional view indicating a positional relationship between the TFT substrate and an opposing substrate of the conventional liquid crystal display device. Note that the liquid crystal display devices shown in FIGS. 1 and 2 are not the same one, but are different from each other in, for example, the geometric shape of a source electrode.
In a conventional liquid crystal display device, a liquid crystal layer 33 is formed between transparent substrates 1 and 21. Hereinafter, the sides of the transparent substrates 1 and 21 on which the liquid crystal layer 33 is disposed will be referred to as “inner side” and the sides thereof on which the liquid crystal layer is not disposed will be referred to as “outer side.”
On an inner side surface of the transparent substrate 1 a gate electrode 3 connected to a scanning line 2 and a gate insulation film 4 covering the gate electrode 3 are formed. A semiconductor layer 5 is formed on the gate insulation film 4 so as to face the gate electrode 3. Further, a drain electrode 7 and a source electrode 8 are formed on the gate insulation film 4 interposing the semiconductor layer therebetween, thereby constituting a thin film transistor (TFT) 10. Note that the drain electrode 7 is connected to a data line 6 that extends in a direction perpendicular to the scanning line 2.
A passivation film 9 and an overcoat layer 11 are formed covering the above-described components. Furthermore, a contact hole 12 is formed in the passivation film 9 and the overcoat layer 11, and a pixel electrode 13 connected to the source electrode 8 via the contact hole 12 in each pixel is formed on the overcoat layer 11. In order to prevent leakage of light through the peripheries of the pixel electrode 13, the pixel electrode 13 is formed to overlap two adjacent scanning lines 2, 2 and two adjacent data lines 6, 6 in its peripheral portions when viewing the substrate from a position vertical to the transparent substrate 1, the two adjacent scanning and two adjacent data lines surrounding the pixel corresponding to the pixel electrode 13. Moreover, an alignment film (not shown) is formed on the pixel electrode 13, thereby constituting a TFT substrate 31.
On the other hand, on an inner side surface of the transparent substrate 21 are formed a color filters 22 corresponding to each of colors to be displayed and a black matrix 23. Further, a common electrode 24 and an alignment layer (not shown) are successively formed on the transparent substrate 21 on an inner side with respect to the color filter and the black matrix, thereby constituting an opposing substrate 32.
It should be noted that in FIG. 2, the source electrode 8 is formed to extend in the vicinity of a pixel adjacent to the pixel containing the source electrode 8. And the scanning line 2 is constructed such that a straight-line portion of the scanning line extending in one direction and another portion thereof projecting from the straight line portion in a direction opposite to the direction in which the gate electrode 3 projects from the straight-line portion and overlapping the source electrode 8 via the gate insulation film 4 are formed to constitute the scanning line. On the other hand, in FIG. 2, the source electrode 8 is formed only in the vicinity of the gate electrode 3 of the TFT 10. As described above, the pixel constructions shown in FIGS. 1 and 2 are a little bit different from one another in terms of structure of a source electrode. That is, a portion of the scanning line 2, which overlaps the source electrode 8, is formed to securely make capacitance between the source electrode 8 and the scanning line 2 large enough to suppress undesirable fluctuation of potential of the source electrode 8 thereby preventing the displayed image from flickering.
A method of manufacturing a conventional TFT substrate will be explained below. FIGS. 3A to 3C and FIGS. 4A, 4B are cross sectional views of a conventional TFT substrate, illustrating a method for manufacturing the conventional TFT substrate in order of process steps. Note that a portion of a pixel shown in each of FIGS. 3A to 3C and FIGS. 4A, 4B corresponds to the portion of a pixel shown in FIG. 2.
As shown in FIG. 3A, first, on a transparent substrate 1 are selectively formed a scanning line 2 (refer to FIGS. 3 and 4) and a gate electrode 3. Then, a gate insulation film 4 is deposited using CVD on an entire surface of the transparent substrate 1. Thereafter, on the gate insulation film 4 a semiconductor layer 5 is formed. Subsequently, a data line 6, a drain electrode 7 and a source electrode 8 are formed on the gate insulation film 4.
As shown in FIG. 3B, a passivation film 9 is formed using plasma CVD on an entire surface of the transparent substrate 1.
As shown in FIG. 3C, an opening 9a is formed in the passivation film 9 so as to reach the source electrode 8.
As shown in FIG. 4A, an organic film, as an overcoat layer 11 having an opening 11a therein that reaches the opening 9a, is formed on the passivation film 9. The openings 9a and 11a constitute a contact hole 12.
As shown in FIG. 4B, a pixel electrode 13 connected to the source electrode 8 via the contact hole 12 is formed on the overcoat layer 11.
Thereafter, an alignment film (not shown) is formed covering the pixel electrode 13, thereby completing formation of TFT substrate.
In general, when a TFT is heated, on-resistance of the TFT is lowered while a ratio of off-resistance thereof with respect to on-resistance thereof is lowered and therefore, heating of TFT has been avoided.
According to the above-described method for manufacturing a conventional TFT substrate, although it is possible to obtain an active matrix type liquid crystal display device capable of displaying relatively high-quality images, it has been found that there are problems associated with the above-described display device. For example, non-uniformity in a display is observed or the contrast in the display degrades after using the device over a long period of time.